HOUSTON (Reuters) - The delayed coking unit at Exxon Mobil Corp’s 362,300 barrel per day (bpd) Beaumont, Texas, refinery will at best run at reduced production levels for up to a month, sources familiar with plant operations said on Friday.

An Exxon sign is seen at a gas station in Illinois, U.S., October 27, 2016. REUTERS/Jim Young

An Exxon spokeswoman said partial production has been restored at the refinery, which was shut on Aug. 30 by heavy rainfall from Hurricane Harvey.

Two drums of the eight-drum, 45,000 bpd coker are filled with petroleum coke sludge and need to be cleaned before they can resume output, the sources said.

Most production units at the refinery are expected to reach their best possible capacity by early next week, the sources said.

Floodwaters from Harvey’s intense, heavy rain disrupted steam supply to the two coker drums causing them to fill with a petroleum coke sludge, the sources added.

Some lines going to the coker also were plugged by the sludge, according to the sources.

Cokers convert residual crude into motor fuel feedstocks or petroleum coke, which can be used as a coal substitute.

To make petroleum coke, residual crude is spun at high speed under high temperature in long drums. The crude hardens into coke on the walls of the drums, which are pulled out at 12- or 24-hour intervals. Then the coke is cut away from the drums by high-pressure, high-temperature water jets.

The flood waters disrupted the process leading to the sludge, the sources said.

The residual crude comes from the crude distillation units, so if the coker is at reduced output, production on one or both of the Beaumont refinery’s crude units will likely be reduced.